feat(frontend): add admin card to generate thumbnails with polling

Fourth card on /admin/system mirrors the mass-import pattern:
- POST /api/admin/generate-thumbnails to trigger
- 2000 ms polling on /api/admin/thumbnail-status while RUNNING
- processed / skipped / failed counters in the DONE message
- standalone pollInterval so import and thumbnail polling don't
  interfere with each other

Paraglide keys added in de/en/es, mirroring admin_system_import_*.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/messages/de.json

@@ -338,6 +338,13 @@
 	"admin_system_import_status_running": "Import läuft…",
 	"admin_system_import_status_done": "Import abgeschlossen – {count} Dokumente verarbeitet.",
 	"admin_system_import_status_failed": "Fehler: {message}",
+	"admin_system_thumbnails_heading": "Thumbnails erzeugen",
+	"admin_system_thumbnails_description": "Erzeugt Vorschaubilder für Dokumente ohne Thumbnail (z. B. nach dem Massenimport).",
+	"admin_system_thumbnails_btn_start": "Thumbnails erzeugen",
+	"admin_system_thumbnails_btn_retry": "Erneut starten",
+	"admin_system_thumbnails_status_running": "Thumbnail-Generierung läuft…",
+	"admin_system_thumbnails_status_done": "Fertig – {processed} erzeugt, {skipped} übersprungen, {failed} fehlgeschlagen.",
+	"admin_system_thumbnails_status_failed": "Fehler: {message}",
 	"comp_expandable_show_more": "Mehr anzeigen",
 	"comp_expandable_show_less": "Weniger anzeigen",
 	"error_comment_not_found": "Der Kommentar wurde nicht gefunden.",